Electrochemical conversion of polyalcohols to hydrocarbons

ABSTRACT

A method of making hydrocarbons from polyalcohols, such as carbohydrates. The polyalcohols and carbohydrates may be provided from biomass, including paper, cardboard or urban generated paper product waste; wood and wood products, including forest slash and deadfall; agricultural waste; and the like. The polyalcohols and carbohydrates are combined with hydroiodic acid in aqueous solution to form the hydrocarbon and elemental iodine. Hydroiodic acid is then electrochemically regenerated by reducing the elemental iodine in a parallel reaction. A method of electrochemically generating hydroiodic acid from elemental iodine in aqueous solution is also described.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/782,905, filed Mar. 15, 2006.

STATEMENT REGARDING FEDERAL RIGHTS

This invention was made with government support under Contract No.DE-AC51-06NA25396, awarded by the U.S. Department of Energy. Thegovernment has certain rights in the invention.

BACKGROUND OF INVENTION

The invention relates to the conversion of polyalcohols to hydrocarbons.More particularly, the invention relates to conversion of polyalcoholsto hydrocarbons by reaction with hydroiodic acid (HI). Even moreparticularly, the invention relates to conversion of carbohydrates tohydrocarbons combined with the regeneration of HI.

A variety of applications use aliphatic hydrocarbons, includingcombustion fuels for heating, transportation, manufacturing, powergeneration, and the like. As world oil reserves dwindle, however, othermeans of maintaining hydrocarbon production rates at their current levelmust be sought.

Carbohydrates (i.e., biomass) are a potential alternative source ofhydrocarbons for use as fuel oil. The conversion of carbohydrates tohydrocarbons can be accomplished using HI as a reducing agent. Thereaction between HI and carbohydrates leads to the production ofhydrocarbons and iodine (I₂) by first removing a hydroxyl radical fromthe carbohydrate which produces water and leaves the iodine ion attachedto the carbon backbone of the carbohydrate molecule, and secondexchanging the attached iodine for hydrogen which results in thecorresponding hydrocarbon and one I₂ molecule.

The major expense incurred by the carbohydrate conversion processdescribed above is the use of HI. To promote HI recycling, phosphorousacid (H₃PO₃) has been used. This method adds the cost of H₃PO₃, which isconsumed as HI is recycled, to that of the overall process.

The current method for converting carbohydrates does not minimize thecost and effort required to regenerate HI. Therefore, what is needed isa method of converting carbohydrates that incorporates a HI regenerationcycle that is not based on chemical reduction of iodine. What is alsoneeded is a method of regenerating HI that is not based upon chemicalreduction of iodine.

SUMMARY OF INVENTION

The present invention meets these and other needs by providing a methodof making hydrocarbons from polyalcohols, such as carbohydrates. Thepolyalcohols or carbohydrates may be provided from biomass, includingpaper, cardboard, or urban generated paper product waste; wood and woodproducts, including forest slash and deadfall; agricultural waste; andthe like. The polyalcohols or carbohydrates are combined with HI inaqueous solution to form the hydrocarbon and elemental iodine. HI isthen electrochemically regenerated by reducing the elemental iodine. Amethod of electrochemically generating HI from elemental iodine inaqueous solution is also described.

Accordingly, one aspect of the invention is to provide a method ofmaking at least one hydrocarbon. The method comprises the steps of:providing at least one water-soluble polyalcohol to an aqueous solutionin an electrochemical cell; providing a predetermined concentration ofHI; combining the HI with the at least one water-soluble polyalcohol inthe aqueous solution to form the at least one hydrocarbon and elementaliodine; and electrochemically regenerating HI from the elemental iodinegenerated by the reaction of the water-soluble polyalcohol with HIwithin the electrochemical cell.

A second aspect of the invention is to provide a method ofelectrochemically generating HI from elemental iodine in aqueoussolution. The method comprises the steps of: providing anelectrochemical cell, the electrochemical cell comprising an anodiccompartment having an anode disposed therein, a cathodic compartmenthaving a cathode disposed therein, and a cationic membrane separatingthe anodic compartment and the cathodic compartment, wherein thecationic membrane is permeable with respect to protons (H⁺ ions) andimpermeable with respect to HI, elemental iodine, water-solublepolyalcohols, and hydrocarbons; providing the aqueous solutioncontaining elemental iodine to the cathodic compartment of theelectrochemical cell; providing a predetermined potential across thecathode and anode; electrochemically oxidizing water to form oxygen gasand protons (H⁺ ions) in the anodic compartment; diffusing the protonsfrom the anodic compartment through the cationic membrane into thecathodic compartment; electrochemically reducing the elemental iodine toform iodide ions in the cathodic compartment; and reacting the protonswith the elemental iodide ions to regenerate the HI in the cathodiccompartment.

A third aspect of the invention is to provide a method of making atleast one of a hydrocarbon fuel and a hydrogenated fuel. The methodcomprises the steps of: providing an electrochemical cell, theelectrochemical cell comprising an anodic compartment having an anodedisposed therein, a cathodic compartment having a cathode disposedtherein, and a cationic membrane separating the anodic compartment andthe cathodic compartment, wherein the cationic membrane is hydrogenpermeable and impermeable with respect to HI, elemental iodine,water-soluble polyalcohols, and hydrocarbons; providing at least onewater-soluble carbohydrate to an aqueous solution in the cathodiccompartment, wherein the at least one carbohydrate is derived from abiomass; providing a predetermined concentration of HI; combining the HIwith the at least one water-soluble carbohydrate in the aqueous solutionto form at least one of the hydrocarbon fuel and the hydrogenated fueland elemental iodine; providing a predetermined potential across thecathode and anode; electrochemically oxidizing water to form oxygen gasand protons in the anodic compartment; diffusing the protons from theanodic compartment through the cationic membrane into the cathodiccompartment; electrochemically reducing the elemental iodine in thecathodic compartment to form iodide ions; and reacting the protons withthe elemental iodide ions to regenerate the HI in the cathodiccompartment.

These and other aspects, advantages, and salient features of the presentinvention will become apparent from the following detailed description,the accompanying drawings, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart for a method of making hydrocarbons fromcarbohydrates;

FIG. 2 is a flow chart for a method of electrochemically generating HIfrom elemental iodine in aqueous solution; and

FIG. 3 is a schematic representation of a two-compartmentelectrochemical cell in which the conversion of carbohydrates tohydrocarbons is carried out.

DETAILED DESCRIPTION

In the following description, like reference characters designate likeor corresponding parts throughout the several views shown in thefigures. It is also understood that terms such as “top,” “bottom,”“outward,” “inward,” and the like are words of convenience and are notto be construed as limiting terms. In addition, whenever a group isdescribed as either comprising or consisting of at least one of a groupof elements and combinations thereof, it is understood that the groupmay comprise or consist of any number of those elements recited, eitherindividually or in combination with each other.

Referring to the drawings in general and to FIG. 1 in particular, itwill be understood that the illustrations are for the purpose ofdescribing a particular embodiment of the invention and are not intendedto limit the invention thereto.

For the purposes of understanding the invention, the term “hydrocarbon”is intended to mean compounds consisting of carbon and hydrogen. In someinstances, such as where the invention is used to produce saturatedhydrocarbon fuels, the polyalcohols may be unsubstituted, saturated,linear aliphatic polyalcohols such as, for example, sorbitol. It hasbeen demonstrated that conversion of polyalcohols (also known as“polyols”) such as, for example, carbohydrates, to hydrocarbons can bechemically accomplished using HI as the reducing agent. The reactionleads to the production of hydrocarbons and iodine (I₂) in a two-stepprocess. In the first step, one HI molecule removes a hydroxyl radicalfrom the carbohydrate, producing water and leaving the iodine ionattached to the carbon backbone of the carbohydrate molecule. In thesecond step, a second HI molecule exchanges the attached iodine forhydrogen, which results in the corresponding hydrocarbon and one I₂molecule. One example of carbohydrate conversion has been reported inwhich sorbitol, a water-soluble polyalcohol formed from cellulose, wasconverted into hexane, a hydrocarbon having the same number of carbonatoms.

The major expense incurred by the carbohydrate conversion processdescribed above is the use of HI. In the original investigation,phosphorous acid (H₃PO₃) was used to promote recycling. Such a methodadds the cost of H₃PO₃, which is consumed as HI is recycled, to that ofthe overall process. The production of carbohydrates and other polyolsfrom biomass and their subsequent conversion to hydrocarbons aredescribed in the article “The Use of Catalytic Hydrogenation toIntercept Carbohydrates in a Dilute Acid Hydrolysis of Biomass to Effecta Clean Separation from Lignin,” by J. Michael Robinson et al.,appearing in Biomass and Bioenergy, vol. 26 (2004), pp. 473-483, thecontents of which are incorporated by reference herein in theirentirety.

In the present invention, hydroiodic acid is recovered without theaddition of phosphorous acid. Instead, HI is recovered by theelectrochemical reduction of I₂, which is present in solution as adissolved gas, as it is produced in the cathodic side of atwo-compartment electrochemical cell. In this case, the carbohydrateconversion takes place in the cathodic compartment of theelectrochemical cell. The cathodic compartment is separated from theanodic compartment by a cationic membrane that prevents oxidation ofboth polyalcohols and HI at the anode. Oxidation of these species at theanode would lead to I₂ production without reducing the carbohydrates tohydrocarbons. The final products of the conversion reactions will beoxygen gas at the anodic compartment and hydrocarbons in the cathodiccompartment.

Turning to FIG. 1, a flow chart for a method 100 of making hydrocarbonsfrom carbohydrates is shown. In step 110, at least one water-solublepolyalcohol is provided to an aqueous solution in an electrochemicalcell. Such polyalcohols include, but are not limited to, carbohydrates.Although the following description refers to carbohydrates, it isunderstood that the methods described herein are equally applicable toother polyalcohols as well. An electrochemical cell that may be used toperform method 100 is schematically shown in FIG. 3.

Carbohydrates include sugars, starches, and fibers. Non-limitingexamples of sugars include monosaccharides such as glucose, fructose,galactose, and the like; and disaccharides such as sucrose, lactose,maltose, and the like. Non-limiting examples of starches includepolymers of alpha-D-glucose units, such as amylase (20-30%alpha-D-glucose) and amylopectin (70-80% alpha-D-glucose). Fibersinclude cellulose-based (cellulose is a polymer of beta-D-glucose units)polysaccharide fibers. Almost any source of cellulose or materialcomposed of carbohydrates may be used as a source of the at least onecarbohydrate. In one embodiment, the carbohydrate may be derived frombiomass, such as paper, cardboard or urban generated paper productwaste; wood and wood products, including forest slash and deadfall;agricultural waste; and the like. In another embodiment, the biomass isfirst digested, using methods know in the art, to convert at least aportion of the biomass into water-soluble carbohydrates. Thewater-soluble carbohydrates may include at least one sugar. Theproduction of carbohydrates and other polyols from biomass is describedin the reference by J. Michael Robinson et al., cited hereinabove.

In step 120, a predetermined concentration of HI is provided to theaqueous solution in the electrochemical cell. The HI concentration is ina range from about 5 M to about 10 M. The HI and the at least onewater-soluble carbohydrate are then combined, thereby forming thehydrocarbon and elemental iodine (step 130).

In one embodiment, the electrochemical cell has an anodic compartment inwhich the anode is disposed, and a cathodic compartment, in which thecathode is disposed. A cationic membrane separates the anodiccompartment and cathodic compartment. The cationic membrane is permeablewith respect to protons (e.g., H⁺ ions) and impermeable with respect toHI, elemental iodine, water-soluble polyalcohols, and hydrocarbons. Inthis embodiment, the HI and the at least one water-soluble carbohydrateare combined in the cathodic compartment of the electrochemical cell.

HI is then electrochemically regenerated in step 140 by reducing theI₂—which is present as a gas dissolved in solution—generated by theformation of the hydrocarbon. A potential V is provided across thecathode and anode of the electrochemical cell, causing water to beoxidized at the anode of the electrochemical cell, producing oxygen gasand protons (H⁺). Iodine is electrochemically reduced to iodide (I⁻)ions at the cathode. The potential is in the range form about 0.7 voltsto about 2.5 volts. In one embodiment, the potential is about 0.7 volts.The actual potential needed depends on the conductivity of the cationicmembrane and the concentration of electrolyte in the electrochemicalcell. With the potential applied, protons migrate through the cationicmembrane to the cathode region of the electrochemical cell, where theycombine with the iodide ions formed by the reduction of I₂ toregeneratively form HI. The regenerated HI may then be used in thecontinued conversion of carbohydrate. Where the electrochemical cell hasanodic and cathodic compartments separated by a cationic membrane,protons migrate from the anodic compartment through the cationicmembrane to the cationic compartment, where they combine with iodideions generated by the reduction of I₂ to form HI. Because iodine iscontinually recycled to regenerate HI, the cell is not limited by HIconsumption, and can be run continuously (i.e., as long as water-solublepolyalcohol is provided).

In one embodiment, formation of the at least one hydrocarbon by themethod 100 described above is carried out in a two-compartmentelectrochemical cell, shown in FIG. 3. Electrochemical cell 300 includesan anode 320, disposed in an anodic compartment 322, and a cathode 310disposed in a cathodic compartment 312. Potential V is applied acrosscathode 310 and anode 320 to drive the oxidation of water and thereduction of I₂. A cationic membrane 330 separates anodic compartment322 and cathodic compartment 312. In one embodiment, cationic membraneis a proton exchange membrane such as those known in the art, including,for example, poly(perfluoro sulfonic acid) or the like. One non-limitingexample of a poly(perfluoro sulfonic acid) proton exchange membrane isNafion®.

Cationic membrane 330 is needed to separate anionic and cathodicprocesses while ionic contact between anionic compartment 322 andcathodic compartment 312 is maintained. The ionic contact is based onprotons (H⁺ in FIG. 3) flowing through cationic membrane 330 fromanionic compartment 322 to cathodic compartment 312.

The electrical energy needed to convert carbohydrate from biomass intothe analogous hydrocarbon is only 20% of the energy that could beharvested through combustion. Whereas the energy required toelectrochemically recycle the twelve HI molecules that drive the hexaneconversion process of the present invention (assuming sorbitol to be thewater soluble analog for the cellulose raw material) is 810 kJ/mol (194kcal/mol) at perfect yield, the total energy released by the combustionof a representative hydrocarbon, hexane, is 4163 kJ/mol (995 kcal/mol).

The carbohydrate to hydrocarbon electrochemical conversion processdescribed herein can take advantage of the solar energy already storedby photosynthesis in the original production of the carbohydrate. Thus,the conversion process may potentially yield eight times the combustionenergy (per unit of electrical energy stored) that is available throughelectrolysis. The voltage V (FIG. 3) required to promote theelectrochemical conversion is at least about 0.7 volts, which isconsiderably less than the voltage needed to electrolyze water, which is2.06 volts. The electrical energy required to electrolyze water is 397kJ/mol (94.8 kcal/mol). However, the energy that can be retrieved by thecombustion of hydrogen is only 236 kJ/mol (56.4 kcal/mol), or 59% of theelectrical energy invested in the separation.

The invention also provides a method of electrochemically generating HIfrom elemental iodine in aqueous solution. A flow chart for the methodis shown in FIG. 2. An electrochemical cell 300, shown in FIG. 3 anddescribed hereinabove, is provided in step 210. In step 220, an aqueoussolution containing elemental iodine is provided to cathodic compartment312 of electrochemical cell 300, and a potential V is applied acrosscathode 310 and anode 320 (step 230). The potential is in the range fromabout 0.7 volts to about 2.5 volts. In one embodiment, the potential isat least about 0.7 volts. The actual potential needed depends on theconductivity of the cationic membrane 330 and the concentration ofelectrolyte in the electrochemical cell. In one embodiment, thepotential is about 0.7 volts. Water is oxidized at anode 320 in anodiccompartment 322 (step 240), producing oxygen gas and protons (H⁺ ions).In step 260, elemental iodine (I₂), which is present in aqueous solutionas a dissolved gas, is reduced at cathode 310 to form iodide (I⁻) ions.Protons diffuse from anodic compartment 322 through cationic membrane330 to cathodic compartment 312 (step 250) where they react with iodideions to form HI (step 260).

Given that hydrocarbons generated by method 100 are generally eithergases or oils that float on the surface of aqueous chemical solution305, the hydrocarbons may be physically separated from chemical solution305 once conversion of polyalcohols to hydrocarbons has taken place.Hydrocarbons generated by method 100 may be used in various applicationssuch as, but not limited to, combustion fuels for heating,transportation, manufacturing, power generation, and the like. Inaddition to hydrocarbons, method 100 may be used to generate oxygenatedfuels such as, but not limited to, ethanol. Such oxygenated fuels may bemade by halting step 130 prior to removal of all the hydroxyl groupsfrom the saturated polyalcohol. Step 130 may be stopped or controlled bylimiting the amount of HI added in step 120, or by stopping theelectrochemical regeneration of HI (step 150).

Although typical embodiments have been set forth for the purpose ofillustration, the foregoing description should not be deemed to be alimitation on the scope of the invention. Accordingly, variousmodifications, adaptations, and alternatives may occur to one skilled inthe art without departing from the spirit and scope of the presentinvention.

1. A method of making at least one hydrocarbon, the method comprisingthe steps of: a. providing at least one water-soluble polyalcohol to anaqueous solution in an electrochemical cell; b. providing apredetermined concentration of hydroiodic acid; c. combining thehydroiodic acid with the at least one water-soluble polyalcohol in theaqueous solution to form elemental iodine and the at least onehydrocarbon; and d. electrochemically regenerating hydroiodic acid fromthe elemental iodine generated by the reaction of the at least onewater-soluble polyalcohol with hydroiodic acid within theelectrochemical cell.
 2. The method according to claim 1, wherein the atleast one water-soluble polyalcohol includes at least one water-solublecarbohydrate.
 3. The method according to claim 2, wherein the at leastone water-soluble carbohydrate comprises at least one of a sugar, astarch, and a fiber.
 4. The method according to claim 3, wherein the atleast one water-soluble carbohydrate comprises at least one sugarselected from the group consisting of glucose, fructose, galactose,sucrose, lactose, and maltose.
 5. The method according to claim 3,wherein the fiber is a biomass derived from at least one of paper, paperproduct waste, wood, wood products, agricultural waste, and combinationsthereof.
 6. The method according to claim 5, wherein the step ofproviding at least one water-soluble polyalcohol to an aqueous solutionin an electrochemical cell comprises digesting the biomass to convert atleast a portion of the biomass into the at least one water-solublecarbohydrate.
 7. The method according to claim 2, wherein the at leastone water-soluble carbohydrate comprises sorbitol, and wherein the atleast one hydrocarbon comprises hexane.
 8. The method according to claim1, wherein the electrochemical cell comprises an anodic compartmenthaving an anode disposed therein, a cathodic compartment having acathode disposed therein, and a cationic membrane separating the anodiccompartment and a cathodic compartment, wherein the cationic membrane ispermeable with respect to protons and impermeable with respect tohydroiodic acid, elemental iodine, water-soluble polyalcohols, andhydrocarbons.
 9. The method according to claim 8, wherein the step ofcombining the hydroiodic acid with the at least one water-solublepolyalcohol in the aqueous solution to form the at least one hydrocarbonand elemental iodine comprises combining the hydroiodic acid with the atleast one water-soluble polyalcohol in the aqueous solution in thecathodic compartment to form the at least one hydrocarbon and elementaliodine.
 10. The method according to claim 8, wherein the step ofregenerating hydroiodic acid from the elemental iodine comprises: a.providing a predetermined potential across the cathode and anode; b.electrochemically oxidizing water to form oxygen gas and protons in theanodic compartment; c. diffusing the protons through the cationicmembrane from the anodic compartment through the cationic membrane intothe cathodic compartment; d. electrochemically reducing the elementaliodine to form iodide ions in the cathodic compartment; and e. reactingthe protons with the elemental iodide ions to regenerate the hydroiodicacid in the cathodic compartment.
 11. The method according to claim 10,wherein the predetermined potential is less than the voltage needed toelectrolyze water.
 12. The method according to claim 1, wherein thepredetermined concentration of hydroiodic acid is in a range from about5 M to about 10 M.
 13. A method of electrochemically generatinghydroiodic acid from elemental iodine in aqueous solution, the methodcomprising the steps of: a. providing an electrochemical cell, theelectrochemical cell comprising an anodic compartment having an anodedisposed therein, a cathodic compartment having a cathode disposedtherein, and a cationic membrane separating the anodic compartment andthe cathodic compartment, wherein the cationic membrane is permeablewith respect to protons and impermeable with respect to hydroiodic acid,elemental iodine, water-soluble polyalcohols, and hydrocarbons; b.providing the aqueous solution containing elemental iodine to thecathodic compartment of the electrochemical cell; c. providing apredetermined potential across the cathode and anode; d.electrochemically oxidizing water to form oxygen gas and protons in theanodic compartment; e. diffusing the protons from the anodic compartmentthrough the cationic membrane into the cathodic compartment; f.electrochemically reducing the elemental iodine to form iodide ions inthe cathodic compartment; and g. reacting the protons with the elementaliodide ions to regenerate the hydroiodic acid in the cathodiccompartment.
 14. The method according to claim 13, wherein thepredetermined potential is less than the voltage needed to electrolyzewater.
 15. A method of making at least one of a hydrocarbon fuel and anoxygenated fuel, the method comprising the steps of: a. providing anelectrochemical cell, the electrochemical cell comprising an anodiccompartment having an anode disposed therein, a cathodic compartmenthaving a cathode disposed therein, and a cationic membrane separatingthe anodic compartment and the cathodic compartment, wherein thecationic membrane is permeable with respect to protons and impermeablewith respect to hydroiodic acid, elemental iodine, water-solublecarbohydrates, and hydrocarbons; b. providing at least one water-solublecarbohydrate to an aqueous solution in the cathodic compartment, whereinthe at least one water-soluble carbohydrate is derived from a biomass;c. providing a predetermined concentration of hydroiodic acid; d.combining the hydroiodic acid with the at least one water-solublecarbohydrate in the aqueous solution to form at least one of thehydrocarbon fuel and the oxygenated fuel and elemental iodine; e.providing a predetermined potential across the cathode and anode; f.electrochemically oxidizing water to form oxygen gas and protons in theanodic compartment; g. diffusing the protons through the cationicmembrane from the anodic compartment through the cationic membrane intothe cathodic compartment; h. electrochemically reducing the elementaliodine to form iodide ions in the cathodic compartment; and i. reactingthe protons with the elemental iodide ions to regenerate the hydroiodicacid in the cathodic compartment.
 16. The method according to claim 15,wherein the at least one water-soluble carbohydrate comprises at leastone of a sugar, a starch, and a fiber.
 17. The method according to claim16, wherein the at least one water-soluble carbohydrate comprises atleast one sugar selected from the group consisting of glucose, fructose,galactose, sucrose, lactose, and maltose.
 18. The method according toclaim 15, wherein the biomass is derived from at least one of paper,paper product waste, wood, wood products, agricultural waste, andcombinations thereof.
 19. The method according to claim 15, wherein stepof providing at least one water-soluble carbohydrate to an aqueoussolution in an electrochemical cell comprises digesting the biomass toconvert at least a portion of the biomass into the at least onewater-soluble carbohydrate.
 20. The method according to claim 15,wherein the at least one carbohydrate comprises sorbitol, and whereinthe hydrocarbon comprises hexane.
 21. The method according to claim 15,wherein the predetermined potential is less than the voltage needed toelectrolyze water.
 22. The method according to claim 15, wherein thepredetermined concentration of hydroiodic acid is in a range from about5 M to about 10 M.
 23. The method according to claim 15, wherein theoxygenated fuel comprises ethanol.